1. Field of the Invention
The present invention relates to a capacitor microphone capable of providing favorable directivity and frequency characteristic.
2. Description of the Related Art
An output voltage of a capacitor microphone unit is proportional to a displacement of a vibrating membrane. If the capacitor microphone unit is configured as a non-directional microphone unit, the capacitor microphone unit has no frequency dependence at a frequency lower than a resonance point of a vibrating system. However, once directivity is provided for the capacitor microphone unit, an electromotive force applied to the vibrating membrane has frequency dependence.
FIG. 8 shows the most standard configuration of a unidirectional microphone unit. FIG. 8 shows an internal structure of the directional microphone unit. In a cylindrical outer casing 11, a case 12 having a rear plate 12R on one end of a cylindrical body in an axial direction is housed. On the other end of the case 12 in the axial direction, a diaphragm (for example, a thin vibrating membrane; a vibrating section) 13 for closing the open end of the case 12 is provided.
At the location separated from the diaphragm 13 by a predetermined distance toward the rear plate 12R, a back plate 14 including a plurality of holes perforated therethrough is provided. In the approximate center of the back plate 14 in a radial direction, an electrode bar 14a having a predetermined diameter is formed. The electrode bar 14a is provided to extend toward the rear plate 12R so that its end penetrates through the rear plate 12R to form an electrically-conductive terminal 14aa. 
A plurality of through holes 15 are provided in the rear plate 12R. An acoustic resistor 16 made of, for example, cloth is provided on an end of each of the through holes 15, the end being on the outer side of the rear plate 12R. A thin fluid layer 17 is formed by a gap between the diaphragm 13 and the back plate 14. An air resistance of the thin fluid layer 17 is set high enough to prevent a resonance of the diaphragm 13 at a high frequency and not to greatly affect the directivity and the frequency characteristic.
An area on the back side of the back plate 14, which is surrounded by an inner circumference of the case 12, is a hollow space 18. Together with the through holes 15 and the acoustic resistors 16, the hollow space 18 forms a phase-shift circuit to obtain directivity. The frequency dependence of the unidirectional microphone unit controls the vibration of the diaphragm 13 by the acoustic resistors 16. If a resistance value of the acoustic resistors 16 is increased, the directivity is decreased to finally provide no directivity.
In a DC-bias capacitor microphone, a DC-bias voltage is applied through a high resistance between a vibrating membrane and a back plate. In the case shown in FIG. 8, a DC power source for biasing is connected between the diaphragm 13 and the outer casing 11 (not shown) so that the outer casing 11 and the case 12 function as the high resistance.
By a change in capacitance of the microphone unit shown in FIG. 8, a change in voltage occurred between both ends of the high resistance (that is, between the outer casing 11 and the terminal 14aa) is obtained as an electric signal.
Conventionally, as described in Japanese Patent Application Publication No. Sho 60-22897, a so-called MS microphone, which uses a unidirectional microphone unit and a bidirectional microphone unit to obtain a stereophonic sound, has been proposed.